Carbon nanotubes are a relatively new material having a hollow tubular structure composed of carbon atoms. Carbon nanotubes have excellent electrical, magnetic, nonlinear optical, thermal, and mechanical properties, such as possessing a high Young's modulus, a high elastic modulus, and a low density. Depending on their length, diameter, and mode of spiraling, carbon nanotubes can exhibit metallic or semiconductor properties. They are widely used in a variety of diverse fields, such as nanometer-scale electronics, materials science, biological science, and chemistry.
At present, methods for producing carbon nanotubes include an arc discharge method, a pulsed laser vaporization method, and a chemical vapor deposition method. The chemical vapor deposition method generally uses transition metals or oxides as a catalyst to grow carbon nanotubes at high temperature by decomposition of carbon-containing reactive gas. Compared with these two methods, the chemical vapor deposition method is superior in operational simplicity, low cost, and mass production, therefore the chemical vapor deposition method has become widely used.
A typical chemical vapor deposition method for producing carbon nanotubes includes the steps of: providing a substrate coated with a catalyst layer on a surface; putting the substrate in a reaction device; heating the reaction device; introducing a carbon-containing reactive gas and thereby growing carbon nanotubes on the substrate.
However, when using a typical method to produce carbon nanotubes after about 5 to 30 minutes, the rate of precipitation of carbon is greater than that of diffusion of carbon. Thus, the catalyst particles become blocked by accumulation of the decomposed carbon of the carbon-containing reactive gas. Therefore, the carbon nanotubes stop growing at a short length.
What is needed, therefore, is an apparatus and a method for producing carbon nanotubes that can have greater length.